This invention relates to communication systems and, more particularly, to estimation of the impulse response coefficients of a communication channel and of the carrier frequency offset.
In communication systems, for example, digital wireless or the like, received data symbols are corrupted by distortion caused by the communication channel over which they were transmitted and noise. Additionally, the data symbols are also corrupted by carrier frequency offset caused by disparity in the frequencies of a remote transmitter and a local receiver.
The problem is to generate an estimate of the communication channel impulse response coefficients and an estimate of the frequency offset simultaneously in the presence of the data symbol distortion. In prior arrangements, one being the least mean squares (LMS) algorithm, the obtained estimate of the communication channel impulse response coefficients is accurate only in the presence of relatively small frequency offset values. The LMS algorithm is not capable of tracking rapid changes in the communication channel impulse response caused by frequency offset and, consequently, looses its capability to estimate the communication channel impulse response coefficients. This is extremely undesirable in communication systems requiring rapid carrier frequency acquisition and tracking, for example, burst communication systems, e.g., time division multiple access (TDMA) or the like.
Note that in prior arrangements, it was usually required to generate a separate frequency offset value in order to improve the estimation of the communication channel impulse response coefficients or to generate a separate estimate of the communication channel impulse response coefficients in order to improve the frequency offset estimation.
These and other problems and limitations of prior known arrangements are overcome by employing apparatus and/or a process which simultaneously generates estimates of both the communication channel impulse response coefficients and the received signal carrier frequency offset at the same time without either estimation interfering with or hampering of the other estimation. This is realized in an embodiment of the invention by utilizing both received symbols known to the receiver during a training sequence and a received signal to generate simultaneously an estimate of the impulse response coefficients of the communication channel and an estimate of the received signal carrier frequency offset.
In one embodiment of the invention a first estimator and a second estimator are employed to generate the communication channel impulse response coefficients and an estimate of the received signal carrier frequency offset, respectively. Specifically, in the first estimator, (a) data symbols in the training sequence, (b) the magnitude of the received signal, (c) the magnitude of an estimate for the received signal and (d) a step size based on a learning coefficient are employed in a first prescribed relationship to generate the communication channel impulse response coefficients. Simultaneously, in the second estimator, a second prescribed relationship between the received signal and its estimate is employed to generate the estimate of the received signal carrier frequency offset.
In another embodiment of the invention, the estimation process is begun utilizing the first estimator to generate the communication channel impulse response coefficients and, simultaneously, the second estimator to generate the estimate of the received signal carrier frequency offset, with the latter being monitored. When the estimated frequency offset falls below a predetermined threshold value, the generation of the communication channel impulse response coefficients is switched to use a third estimator, for example, a least mean squares (LMS) estimator, while generation of the received signal carrier frequency offset estimation continues simultaneously.